1. Field of the Invention
This invention relates generally to a system and method for heating a fuel cell stack at system start-up and, more particularly, to a system and method for heating a fuel cell stack system start-up that includes electrically shorting the stack and using cathode air as a limiting reactant.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
During low temperature operation, such as below 50° C., a fuel cell stack generally operates with liquid water in the flow channels due to a low water saturation pressure. This liquid water can cause flow distribution problems, freeze start problems and electrode flooding. If the stack temperature was increased, many of these problems could be avoided. If the stack is below freezing, then ice may form in the flow channels, which needs to be quickly melted to liquid water or water vapor at system start-up so that it can be purged out of the flow channels to provide adequate flow distribution. At system shut-down, actions are taken to remove as much of the water from the stack as possible through flushing of liquid water droplets from channels and evaporate drying of the MEAs and diffusion media. However, it is generally not possible to remove as much of the water as desired from the MEAs and diffusion media, especially for low temperature starts.
It is known in the art to use the waste heat generated by a fuel cell stack to bring the system to its operating temperature, which can take a relatively long time because of the inherent efficiency of the fuel cell stack. It is also known to use a heater to heat the stack cooling fluid at system start-up so that the temperature of the stack increases more quickly. This heat put into the system is limited by the size of the cooling fluid heater and the area over which the heat transfer occurs. It is also known to inject hydrogen gas into the cathode air stream to the stack to allow for catalytic combustion of the hydrogen on the catalyst in the cathode side of the fuel cells to provide heat. However, there are limits as to the amount of hydrogen that can be sent to the cathode because of flammability and stack dry-out concerns.